FIG. 1 depicts a conventional heterojunction bipolar transistor (HBT) 10 formed on a substrate 11. The conventional HBT 10 includes a conventional collector 12, a conventional compound base 16, and a conventional emitter 20. In some conventional HBTs 10, the conventional collector 12 is separated from the conventional compound base 16 by a conventional spacer layer 14. In addition, a conventional capping layer 18 may be provided between the conventional compound base 16 and the conventional emitter 20.
In a conventional HBT 10, the conventional compound base 16 is typically formed from a compound layer, generally of doped SiGe or SiGeC. The dopant used for the conventional compound base 16 is boron. In addition, the conventional spacer layer 14 and conventional capping layer 18 are typically undoped silicon. The conventional emitter 20 and the conventional collector 12 are typically doped so that the conventional HBT 10 is an NPN or a PNP transistor.
Although the conventional HBT 10 functions, one of ordinary skill in the art will readily recognize that improved performance of the conventional HBT 10 is desirable. For example, as device speeds are increased, current gains become greatly elevated and breakdown voltages are greatly reduced, which is often undesirable. The rate of stored charge dissipation in the compound base, base-emitter, and collector regions is desired to be kept elevated. The steep, repeatable boron profiles for the highest Ft and Fmax are also desired to be maintained in order to ensure that the performance of the conventional HBT 10 is repeatable. Furthermore, one of ordinary skill in the art will readily recognize that the introduction of certain contaminants can adversely affect the performance of the conventional HBT 10. For example, although oxygen may reduce boron out diffusion and reduce the pinched base resistance RSBi, such oxygen diffusion is uncontrolled and thus is typically regarded as an undesirable contaminant in conventional SiGe and SiGeC HBT devices. For example, oxygen contamination is typically a result of leaks, outgassing, and permeation sources in the process reactor. Such, oxygen contaminants have been shown to reduce minority carrier lifetime, which leads to increased base recombination current and reduced current gain. Similarly, oxygen contaminants have been observed to result in increased base resistance of SiGe when boron is incorporated as the dopant for the conventional base 16.
Accordingly, what is needed is a method and system for improving performance of the conventional HBT 10. The present invention addresses such a need.